Neither fourth year nor I are saying to leave high city pressure water ‘on’ and ‘connected’ during the test. You cannot look for a pressure drop if the pressure is constantly being re-introduced at the supply.
Both fourth year and I are commenting on your original post stating “Pressurize the tubing to 10-15 psig and look for pressure loss over a day.” I also agree with your safety comment related to using high pressure air, and your safety comments about using high pressure water.
We were responding to the “10-15 psig” recommendation. This low pressure does not accomplish much at all whether air or water is used.